The use of catheters to insert and position therapeutic devices in the body has become a widely-used form of treatment for various conditions. Such devices are particularly useful in treating areas where traditional procedures are difficult such as in narrow cranial blood vessels. For example, vaso-occlusive devices such as embolic coils or wires are inserted at sites of aneurysm to occlude blood flow. The decreased blood flow reduces the pressure on the aneurysm and reduces the risk of a ruptured aneurysm. The coil also promotes thrombus formation. Embolic coils and wires can assume different shapes to better occlude a blood vessel. The coils can be coated with various materials to improve thrombogenicity. U.S. Pat. No. 6,723,108 describes some of the characteristics of different shapes of embolic coils. This patent and all other patents and patent application publications identified herein are hereby incorporated herein by reference.
Typically, procedures using a catheter involve inserting the distal end of the catheter into the vasculature of a patient and guiding it to a predetermined delivery site. A vascular occlusion device, such as an embolic coil, is attached to the end of a structure capable of manipulating the therapeutic device. This structure may, for example, be used to push the coil through the catheter and out of its distal end into the delivery site. The coil is then released from the pusher. The small size of some blood vessels requires that mechanism that releases the coil from the pusher be simple and not require complicated equipment. In addition, the release mechanism must accurately and rapidly place the therapeutic device at the determined site. Problems that have been associated with the release of the coil include the force of the coil exiting the delivery catheter causing the coil to overshoot the desired site or dislodge previously deployed coils.
In response to the above mentioned concerns, numerous devices and release mechanisms have been developed in an attempt to provide a delivery system which provides a rapid release or detachment mechanism to release the device at the correct location. One such device is disclosed in Geremia et al. U.S. Pat. No. 5,108,407, which shows a fiber optic cable including a connector device mounted to the end to the optic fiber. An embolic coil is attached to the connector device by a heat releasable adhesive. Laser light is transmitted through the fiber optic cable to increase the temperature of the connector device, which melts the adhesive and releases the embolic coil. One drawback to using this type of system is the potential risk of melted adhesives contaminating the blood stream.
Yet another embolic coil positioning and delivery system is described in Saadat et al. U.S. Pat. No. 5,989,242, which discloses a catheter having a shape memory alloy connector attached to the distal end of the catheter. The connector includes a socket having a pair of spaced-apart fingers which are responsive to a change in temperature. The fingers are bent towards each other and hold a ball which is connected to an end of an embolic coil. The connector absorbs laser light transmitted through an optical cable and transforms the light into heat energy. The heat energy raises the temperature of the connector and opens the fingers, thereby releasing the embolic coil. This type of ball and socket connection is rigid and causes the catheter to be stiff, making it difficult to guide the catheter through the vasculature of the body.
U.S. Patent application publication 2005/0113864 A1 by Gandhi et al describes an apparatus for the release and deployment of a therapeutic device where the therapeutic device is secured to the distal end of a pusher by a collar. When heated, the collar alters its configuration and releases the therapeutic device into the vasculature. The collar can be formed from shape memory metals or from thermoplastic polymers. In another embodiment found in the same disclosure, the therapeutic device is secured to the pusher by a connector fiber that can be broken by heating, releasing the therapeutic device. The connector fiber can be formed from a thermoplastic material or a biodegradable material that degrades or decomposes with heating. One difficulty associated with these arrangements is that the material forming the collar or fibre may fragment or dissolve when heated, releasing materials into the bloodstream. The Gandhi et al disclosure addresses this by performing the heating step completely within a catheter such that the pusher and therapeutic device become disengaged within the catheter. This approach is problematic because it may reduce the ability of the pusher to manipulate the therapeutic device to precisely the correct location in the vasculature.
In keeping with the invention, therefore, a need remains for a therapeutic device delivery apparatus which uses material that changes to effect therapeutic device release while remaining totally encapsulated, which has a reliable operating principle and is simple to use but still provides excellent control over the therapeutic device during the process of inserting and releasing it in the vasculature.